A Conversation with Jacob Nachmias.

We are sad to report that Professor Jacob (Jack) Nachmias passed away on March 2, 2019. Nachmias was born in Athens, Greece, on June 9, 1928. To escape the Nazis, he and his family came to the United States in 1939.

Shape and size discrimination compared.

Observers presented with pairs of figures differing in area (SIZE) or aspect ratio (SHAPE) spontaneously make use of both height and width differences. whether or not they are forced to do so by between-interval jittering or even instructed to do so. SHAPE discrimination is considerably better than SIZE discrimination.

Judging spatial properties of simple figures.

Discrimination was measured for height, area, and aspect ratio of ovals and rectangles. Random jittering of the orthogonal property (width, aspect ratio, and area) was used to control the observers' criterion. Weber fractions for aspect ratio were consistently lower than those for area, and about the same as those for height.

The role of virtual standards in visual discrimination.

Psychometric functions were measured in various visual discrimination tasks involving test stimuli whose values spanned a standard stimulus. In two-interval trial blocks, the standard was either always in the first or always in the second interval, or appeared randomly in either interval. In one-interval blocks, the standard stimulus was never presented.

Contrast discrimination with and without spatial uncertainty.

The main purpose of these experiments was to examine in detail how successfully the uncertainty reduction explanation accounts for the effects of spatial cues in a temporal forced-choice contrast discrimination task in which any one of four well-separated Gabor patches is incremented. In preliminary experiments, it was shown that in the absence of uncorrelated contrast jitter, observers could use either of at least two different decision strategies, whereas in the presence of random contrast jitter, their response was based on the largest contrast value in the two intervals of a trial. Independent spatial cues were used in each interval to minimize the likelihood of eye movement artifacts.